The present invention relates to a fitting for attachment to an end of a thin walled metal tube. More particularly, it relates to that type of fitting which is attached to the tube by expanding the tube within its interior.
Metal tubing is used in critical applications for the conveyance of fluid under pressure. In many such applications, the rupture of the metal conduit can result in a major catastrophe. This is particularly true in the aerospace industry where the tubing is used to convey fuels and hydraulic control fluids. For example, not too many years ago, an experimental aircraft was lost because of hydraulic line failures in its first and second primary flight control systems followed by a failure in its third back-up system. In that case, the failure was traced to the fracturing of metal tubing used to convey the hydraulic control fluid. Metal tubing was used since it has relatively high strength in relation to its weight and, more importantly for an hydraulic system, resists volumetric expansion and contraction when called upon to handle pulsating fluid pressures. Unfortunately, metal suffers from fatigue, tends to develop cracks when subjected to certain regimes of vibration, and is susceptible to chafing induced failure. Experience has also shown that metal tubing is liable to develop cracks where the tube enters an end fitting. Flexural strength testing in accordance with paragraph 4.7.7 of Specification MIL-F-18280C of aluminum alloy tubes secured to end fittings of the general type disclosed in U.S. Pat. No. 3,711,132, (issued Jan. 16, 1973 to Harvey R. Nickerson and assigned to the same assignee as the present application) revealed that the tubes most frequently experience circumferential fractures within an axial zone ranging from about 0.06 to 0.1 inches inside the fitting envelope although occasional fractures occur just outside the fitting.
Said Nickerson patent describes a fitting assembly which overcame certain fitting related fatigue failure problems encountered with titanium alloy conduits. For the purpose of extending the flex life of such conduits said patent disclosed and claimed a modified fitting wherein the axially spaced circumferential grooves within the bore thereof to which the metal tube is attached by radial expansion are provided with a special contour. The group of grooves nearest the exterior of the fitting is provided with inclined sidewalls, while the inner adjacent group is provided with perpendicular sidewalls. Such fittings have been demonstrated to provide improved performance on aluminum alloy lines as well as on stainless steel and titanium lines. Nevertheless, the performance with respect to aluminum alloy lines still leaves much to be desired.
In copending application Ser. No. 760,108 of Irving D. Press, filed Jan. 17, 1977 for "A Fluid Conduit Assembly", and assigned to the same assignee as the present application, there is disclosed a fluid conduit assembly provided with at least two snug-fitting concentric pipes each of which is independently impervious to fluid. End fittings are secured to each end of the conduit and have separate portions joined, respectively, to a different one of both of the pipes with a separate fluid tight seal therebetween. The outer pipe is metallic while the inner pipe is non-metallic. It was disclosed in said application that the non-metallic liner had a response to mechanical vibration which imposed a damping factor on vibration of the metal conduit. It was pointed out that the non-metallic pipe appeared to damp vibration of the metal pipe to such an extent that at least when the metal pipe is in the form of tubing, substantial increase in flex life is obtained before fracture or fatigue failure of the metal tube.